Lenses made of high refractive index and low dispersion glass make it possible, by being combined with a lens made of hyper low dispersion glass, to downsize an optical system while correcting chromatic aberration, and thus the lenses occupy a very important place as an optical element constituting an imaging optical system and projection optical system such as a projector.
In Literature 1 (Japanese Unexamined Patent Publication (KOKAI) Showa No. 60-33229), which is expressly incorporated herein by reference in its entirety, high refractive index and low dispersion glass having a refractive index of 1.90 to 2.10, and an Abbé number νd of 22 to 35 is disclosed although the glass is not an optical element material for an imaging optical system or a projection optical system.
In contrast, in Literature 2 (Japanese Unexamined Patent Publication (KOKAI) Showa No. 60-131845) or English language family member U.S. Pat. No. 4,584,279, which are expressly incorporated herein by reference in their entirety, optical glass having a refractive index equal to or higher than 1.90, and an Abbé number equal to or higher than 25 is disclosed.